Carbon dynamics in a marsh-influenced marine-dominated ecosystem
A combination of global climate change, local anthropogenic pressures, and naturally occurring processes have impacted biogeochemical cycling in coastal systems. Here, a coastal estuarine ecosystem in North Carolina is studied in order to determine spatial relations, seasonal changes, and overall fluxes of carbon, as well as the influences of these factors on the biogeochemistry of the system as a whole. Partial pressure of carbon dioxide (pCO2), percent dissolved oxygen (DO), particulate organic carbon (POC), total dissolved inorganic carbon (DIC), total alkalinity (TA), and carbon isotopes of organic and inorganic carbon—amongst additional data—were collected from numerous study locations in the Cape Lookout region of North Carolina in April 2017, October 2017, April 2018, June 2018, and October 2018. Carbon isotopes of POC ranging between ‑30 and -17.79‰ coupled with a decreasing trend in C/N values moving down-estuary indicate that the organic carbon in the system is mainly sourced from upland vascular plant and agricultural inputs, with a small influence from in-estuary Spartina marsh grasses. The majority of the estuary was over-saturated with CO2 compared to the atmosphere during all seasons, with the marsh-creek Smyrna Creek consistently exhibiting the most extreme pCO2 values, peaking at 14606 µatm in the head of the creek in June 2018. Some estuarine sites were occasionally undersaturated in CO2, likely from local phytoplankton blooms occurring during spring and summer. Carbon flux from these three creeks into Jarrett Bay is evident, as is further flux of CO2 through the sound and out into the ocean where the CO2-saturated estuarine waters combine with the less CO2-rich marine waters to produce ocean values of ~625 µatm. TA values throughout the system range from 1872–2342 µmol kg-1, excluding Smyrna and Williston marsh-creeks which exhibited anomalous TA in several different seasons. Omitting these two creeks, the remainder of the system shows an increasing spatial TA trend moving down-estuary over the salinity gradient with the lowest values in Jarrett Bay and the highest values in the ocean. Due to seasonal mixing trends, DIC concentration increased down-estuary in the Summer and Spring and decreased over the salinity gradient in the Fall; however, the head of Smyrna Creek typically exhibited notably high DIC compared to the rest of the system, as CO2 is the main contributor to DIC within the salt marsh. Plotting DIC against TA indicates that inorganic carbon likely originates from a combination of sulfate reduction, denitrification, CO2 invasion, and aerobic respiration. Calculations of air–sea CO2 flux indicate that the estuarine waters as a whole are a significant source of CO2 to the atmosphere with an average air–sea CO2 flux of 13.4 mmol m-2 day-1.